Summary: In APOC3*S1/APOC3*S1 subjects, replacement of the SFA diet by the NCEP-1 diet resulted in an increase in basal glucose and insulin concentrations (for glucose, P < 0.001; for insulin, P < 0.008). Also in this group, consumption of the MUFA diet resulted in the lowest basal glucose concentrations (MUFA diet compared with SFA diet, P < 0.001; MUFA diet compared with NCEP-1 diet, P < 0.000001). Basal insulin concentrations were also significantly lower with the MUFA diet than with the NCEP-1 diet (P < 0.0002). In carriers of the S2 allele, basal glucose concentrations were significantly lower after the MUFA diet than after the NCEP-1 diet (P < 0.006). There were no significant differences in this subject group in basal glucose concentrations after the NCEP-1 and SFA diets.